Terahertz-Erkennung energetischer Materialien : spektroskopische Analysen und technologische Perspektiven bildgebender Systeme

In recent years, there is a growing demand for technology to identify and recognize materials relevant to security. The terahertz (THz) frequency range, situated between microwaves and infrared in the electromagnetic spectrum, has certain properties making it predestined for security applications. Many materials, e.g. energetic materials (explosives), exhibit resonant absorptions, so called spectral fingerprints, in the THz-frequency range which facilitate their recognition. In combination with the transparent nature of many clothing- and packaging materials to THz-radiation, the application of THz-technology for the recognition of hidden explosives is obvious. In order to successfully recognize energetic materials with THz-technology, requirements in two areas have to be fulfilled, namely suitable THz-technologies for material detection and detailed spectroscopic THz-analyses of energetic materials.

This thesis investigates both areas by evaluation of suitable detector technologies for THz material recognition as well as an analysis of spectroscopic experiments on a selection of interesting energetic materials. THz time-domain spectroscopy setups in transmission and reflection, which where developed in the context of this thesis, are used to analyze the energetic materials. Spectroscopy in transmission mode, facilitating a very high bandwidth from 0,2 - 6,6 THz, is used to determine the absorption-coefficient as a quantitative measure and enabled the author to measure a clear resonance of the explosive RDX at 4,2 THz for the first time. The variability of spectral fingerprints due to additives, inhomogeneities and preparation of explosives are investigated via THz-spectroscopy. Results show that an inhomogeneous broadening of a resonance can occur due to the preparation of the energetic material. This is a challenge for THz-based recognition of explosives.

Furthermore, several THz-imaging technologies are investigated regarding their respective potential for THz explosives recognition. A realtime THz-imaging system based on a row of 32 sub-harmonic mixers, developed in cooperation with project partners, as well as a system based on uncooled antenna-coupled thermocouples, both operating at 800 GHz, are characterized. THz-imaging on objects relevant to security is demonstrated using different THz-imaging technologies.
The outcome of THz-imaging of samples of explosives is their recognition based on resonant absorptions using different THz-imaging technologies. In this thesis, the THz-recognition of resonances of explosives using thermocouples as the THz power detector is demonstrated globally for the first time. To conclude, the potential of the THz-recognition of energetic materials is discussed and an assessment of future directions for research is given.